Increasing the power output of a given size internal combustion engine, e.g., a diesel engine, by supercharging has been common practice for decades. Turbocharging has evolved as a preferred method because of its utilization of exhaust gas energy to drive a compressor rather than mechanically connecting the compressor to the engine. Turbocharging results in lower fuel consumption, higher power output potential, and compensation for air density loss when engines are operated at high altitude.
The capability of modern engines to produce more power from a given cylinder displacement has been steadily increased due to engineering innovation and development, and modern engines can utilize higher charge air pressures than a single turbocharger can provide. Thus, a number of high specific power diesel engines employ two turbochargers with their turbines and their compressors connected in series. In a typical arrangement of series turbochargers, one turbocharger is mounted on the exhaust manifold and comprises a high pressure stage, and a second turbocharger comprises a low pressure stage. The turbine of the high pressure stage receives exhaust gas from the manifold and the low pressure stage turbine receives exhaust gas from the high pressure stage and discharges it to the atmosphere. The low pressure stage compressor takes in air from the atmosphere, compresses it, and delivers it to the high pressure stage compressor, sometimes through a charge air cooler. The high pressure compressor stage accomplishes a second stage of charge air compression before delivering the charge air to the intake manifold. The two turbochargers in series present a complicated and expensive means of supplying highly compressed air to an internal combustion engine.
In the near future, heavy-duty engines will be required to meet lower levels of nitrogen oxide (NOx), hydrocarbon (HC) and particulate emissions. Proposals for future emissions regulations are calling for stricter nitrogen oxide while keeping particulate matter standards at their current level. In June of 1996, the EPA proposed a plan for reducing pollution from heavy trucks, which calls for NOx+HC emissions of 2.5 G/BHP-HR and particulate matter of 0.10 G/BHP-HR by 2004.
One method of reducing NOx emissions is exhaust gas recirculation, a technique used in some light-duty diesels and in passenger car gasoline engines. Exhaust gas recirculation (frequently referred to as "EGR") reduces NOx in internal combustion engines by diluting the charge air and depressing the maximum temperature reached during combustion. However, a detrimental effect of EGR is a resulting increase in particulates. Government regulations dictate that particulate emissions must be reduced to a level of 0.10 G/BHP-HR by the year 2004.
Particle traps have been used in dealing with the insoluble diesel particulate problem. This method of diesel exhaust after treatment has been traditionally characterized by high cost and low reliability. Recent developments in passively regenerated traps using fuel additive catalyzing agents have emerged as a lower cost alternate to conventional active regeneration trap systems.
In turbocharged heavy-duty diesel engines it is sometimes difficult to introduce EGR into the intake manifold because turbocharged intake manifold pressures are usually greater than exhaust system pressures. To circumvent this problem exhaust gas has been intercepted at a point upstream of the engine turbocharger where pressure is generally higher than that of the intake manifold. This approach is commonly referred to as high pressure loop (HPL) EGR.
While high pressure loop EGR applied to several heavy-duty diesel engines has been effective in reducing NOx to the 2.0 G/BHP-HR., diverting exhaust gas for EGR upstream of the turbocharger turbine reduces the exhaust gas energy available to drive the turbocharger. The penalty associated with such a system, is an increase in fuel consumption and an associated increase in particulate matter emissions. With a trap-based, after-treatment system, control of the particulate matter may be achieved; however, the fuel consumption penalty remained unresolved.
Another EGR configuration preserves turbocharger performance by supplying exhaust gas for recirculation from a point downstream of the turbocharger. At this location, exhaust gas pressure is at a lower level than that of the intake manifold and the EGR gas can be introduced in the system upstream of the turbocharger compressor. The pressure difference in such systems can be generally adequate for EGR flow rates needed to reduce NOx to the 2.0 G/BHP-HR level. This configuration is known as the low pressure loop (LPL) EGR system.
The advantages of the LPL EGR system over the HPL EGR system include:
1) Lower fuel consumption from that of the HPL configuration as a result of better turbocharger performance than that of the HPL configuration. PA1 2) With the presence of a particulate trap, the LPL EGR supplies filtered exhaust with possible improved engine durability. PA1 3) Because exhaust gas downstream of the trap is cooler than that provided from upstream of the turbocharger (as in the HPL case), LPL EGR would have a higher heat absorbing capacity for rates similar to those of the HPL EGR rates. PA1 4) With cooler EGR there is a possibility of reducing the size of the EGR cooler and providing a more compact unit. PA1 5) Reducing the EGR cooling requirement may help prevent exhaust system condensation and potential erosion of the turbocharger compressor wheel. PA1 6) EGR and fresh charge air mixing may be improved by introducing the mixture upstream of the turbocharger compressor.
Diesel engines have an excellent appetite for the EGR at low idle and no load engine condition where they experience extremely high air-to-fuel (A/F) ratios relative to those of the gasoline engine. At peak torque and rated speed/full load diesel engine conditions, typical A/F ratios are 25 to 30:1, respectively. Therefore, it is desirable to avoid employing EGR at these conditions to reduce or prevent smoke formation.
Motor-assisted turbochargers have been used to improve internal combustion engine performance by supplementing the energy of the exhaust gas at low engine speed with electrical energy applied to an electric motor that assists the turbocharger turbine in driving the turbocharger charge air compressor. When there is a need for engine acceleration from low speed or an increased engine load, the electric motor can be energized and respond by increasing the turbocharger compressor's rotational speed, providing additional boost, and thus improving the engine's response at low speed and to increased loads. As engine speeds increase, for example, beyond peak torque speed, the engine turbocharger can develop the high boost required for the proper engine performance and low exhaust emissions, and the electric motor can be de-energized.